The shoulder joint is one of the largest joints in the human body and has the greatest range of motion of any joint. The shoulder joint is the area where the head of the humerus bone joins the scapula. While the shoulder joint is considered a ball and socket joint, the socket is relatively shallow. The shallow socket contributes to the joint's range of motion. Other bones that play a crucial role in the functionality of the shoulder include the acromion and coracoids process that project from the scapula and the clavicle that is connected to the acromion by the acromioclavicular ligament.
A variety of soft tissues holds the shoulder joint in place and allow the wide range of motion. These tissues include the rotator cuff which comprises several muscles and tendons surrounding the shoulder joint, bursae which are sacs of fluid that provide protection and lubrication within the rotator cuff, and the glenoid labrum which is a fibrocartilaginous rim around the socket in the scapula. Additionally, many other ligaments, tendons, and muscles assist in the function of the shoulder joint.
Because of the loose fit between the humerus and the scapula, the wide range of motion of the joint, and the general abuse the joint receives, shoulder injuries are common. Strains and tears of ligaments and tendons, and tears of the tissue comprising the rotator cuff are frequent occurrences for people with an active lifestyle or job, as well as the elderly. Injuries may also occur due to a single event such as lifting a heavy object or an accident.
Each of these injuries typically result in a significant amount of pain, particularly when moving the arm. The most intense pain may occur when the arm is moved towards the extremes of motion and soft tissue is impinged between the head of the humerus and the acromion.